The monoadducts and crosslinks formed by photochemical cycloaddition of psoralens to DNA are isolated by enzymatic digestion of DNA after reaction with psoralens. By high pressure liquid chromatography four monoadducts to thymine and one crosslink have been resolved and are being isolated in sufficient quantity for structure analysis by high resolution nuclear magnetic resonance. Very high specific activity tritiated psoralen derivatives are being synthesized for kinetic studies of the photoaddition of these psoralens to DNA. Nucleic acid duplex-nucleic acid duplex crosslinking reagents are now being synthesized containing a radiolabel (tritium) in order to enhance our efforts to optimize the crosslinking condition for this photochemistry.